Regenerators are an integral part of cryocoolers to reach low temperatures between 4 K and 20 K (approximately 270 to 250 K below room temperature) regardless of the refrigeration technique employed; e.g., regardless of whether the known Gifford-McMahon, Stirling, pulse tube, etc. cooling technique is employed. A two stage Gifford-McMahon cycle cryocooler or refrigerator used to reach extremely low temperatures, such as approximately 10 K, without a liquid refrigerant is discussed in U.S. Pat. No. 5 186 765. For discussion of other cryocoolers, see books entitled "Cryogenic Heat Exchangers", Plenum Press, New York, 1997, by R. A. Ackerman and entitled "Cryocoolers Part 1: Fundamentals", Plenum Press, New York 1983 by G. Walker.
One important property of a highly effective regenerator is that the regenerator material should have a large volumetric heat capacity. Most commercial regenerators today employ bronze or stainless steel screens or spheres to cool down to approximately 100 K, and lead (Pb) spheres to cool below 100 K, with 10 K being the no heat load low temperature limit because the heat capacity of lead becomes extremely low at that temperature. Sometimes a combination of bronze or stainless steel and lead are used for cooling below 50 K with a layered regenerator bed for a single stage refrigerator. Or, a two stage refrigerator is used with a bronze alloy and stainless steel materials used in the high temperature stage and lead (Pb) used in the low temperature stage as a result of the heat capacity of lead not decreasing as quickly as that of the other materials below 100 K. Above 100 K, most metallic, non-magnetic materials have the same molar heat capacity, reaching the DuLong-Petit limit of 3R, where R=8.314 J/mol K is the universal gas constant. In general, the higher the heat capacity of the regenerator bed material, the greater the cooling power of a cryocooler, all other parameters being equal.
The potential use of lanthanide intermetallic compounds, which exhibit low magnetic ordering temperatures (e.g. less than 10 K), as cryogenic magnetic regenerator materials (refrigerant or cold accumulating materials) was pointed out nearly 25 years ago by Buschow et al. in an article entitled "Extremely Large Heat Capacities between 4 and 10 K, Cryogenics, vol. 15, (1975), pages 261-264. However, a practical lanthanide regenerator material was not developed and put into use until about 15 years later when the use of Er.sub.3 Ni (an intermetallic compound) as a low temperature stage regenerator material in a two-stage Gifford-McMahon cryocooler was proposed by Sahashi et al. in "New Magnetic Material R.sub.3 T System with Extremely Large Heat Capacities Used as Heat Regenerators", Adv. Cryogenic Eng., vol. 35, (1990), pages 1175-1182 and by Kuriyama et al. in "High Efficient Two-Stage GM Refrigerator with Magnetic Material in Liquid Helium Temperature Region", Adv. Cryogenic Eng., vol. 35, (1990), pages 1261-1269.
These articles proposed the replacement of the lead (Pb) lower temperature stage regenerator material with Er.sub.3 Ni intermetallic compound material. Replacement of the lead lower stage regenerator material with Er.sub.3 Ni material (an intermetallic compound) permitted improved cooling to approximately 4.2 K instead of the approximately 10 K achievable with the previously used lead lower stage regenerator material with a reasonable refrigeration capacity at the lowest temperature. This improvement in cooling (i.e. to approximately 4.2 K) is attributable to the significantly higher heat capacity of Er.sub.3 Ni than lead below 25 K (the heat capacity of lead becomes negligible below 10 K).
The Gschneidner and Pecharsky U.S. Pat. No. 5,537,826 issued Jul. 23, 1996, describes an improved regenerator for the low temperature stage (e.g. below 20 K) of a two stage Gifford-McMahon cryocooler. The patented regenerator comprises intermetallic compounds Er.sub.6 Ni.sub.2 Pb, Er.sub.6 Ni.sub.2 (Sn.sub.x Ga.sub.1-x), where x is greater than 0 and less than 1, and Er.sub.6 Ni.sub.2 Sn as a regenerator component.
An object of the present invention is to reduce the cost and to improve the reliability, efficiency and increase the cooling power of a cryocooler at both the low and high temperature ranges or stages, for example, from about 10 K up to 100 K, more generally from approximately 300 K to approximately 10OK.
Another object of the present invention is to utilize ductile magnetic rare earth (lanthanide) based solid solution alloys, which can be easily fabricated into tough, non-brittle, corrosion resistant spherical powders, or thin sheets, or thin wires, or porous monolithic forms (such as cartridges), as the regenerator material.
Another object of the present invention is to provide a cryocooler with a regenerator having significantly higher heat capacity than the aforementioned previously used regenerator materials and combinations thereof, such as bronze, stainless steel, and lead.